Conventionally, a phase cutoff relay (phase relay) has been provided between a motor drive circuit and an electric motor to suppress the generation of electric brake in the electric motor, for example, due to the failure of an inverter circuit or the like. When a semiconductor relay using a semiconductor device such as a MOSFET is employed for this phase relay, it is possible to make the product small and to increase the reliability thereof. However, since the semiconductor relay is largely limited by the withstand voltage, it is necessary to protect the semiconductor relay from surge voltage generated due to the inductance of the electric motor at the time of phase cutoff.
Therefore, for example, in Patent Document 1, a resistor is connected in parallel with a semiconductor relay to protect the semiconductor relay. When current is passed through the resistor in a state in which the semiconductor relay is turned off, it is possible to reduce a difference in potential applied to the semiconductor relay so as not to exceed the withstand voltage.